1. Field of the Invention
The present invention relates to an image forming apparatus for exposing a photosensitive recording medium to light using a mask member, and more particularly to a sensing unit for sensing the mask member provided in a mask member positioning unit of the image forming apparatus, a color image being obtained by exposing to light the photosensitive recording medium which is closely contacted with the mask member on which an image information is formed.
2. Description of Related Art
An image forming apparatus is known which uses, for example, as in U.S. Pat. No. 4,810,614, light transmitting sheets (hereinafter referred to as mask members) having images thereon in black that are obtained by taking out red, green and blue components of a colored original image. With the image forming apparatus, photosensitive recording paper, to which color capsules are applied, is sequentially exposed to light using the three mask members to form an image at an exposing table.
FIG. 6 is a sectional view showing a portion of an image forming apparatus of the above-mentioned type in the area of the exposing table. A mask member 115 for red, another mask member 115 for green and a further mask member 115 for blue, all of which are formed by a laser printer or the like (not shown), are inserted one by one into an inlet port (not shown) and transported to an exposing table 102 by a pair of rollers 132 through mask member guides 124, 125 in the apparatus shown in FIG. 6. The mask member 115 arrives at a predetermined exposure position.
A photosensitive recording medium 106, such as that disclosed in U.S. Pat. No. 4,399,209, is arranged above the exposing table 102. Then, an exposure operation is performed while the photosensitive recording medium 106 and the mask member 115 for red transported to the exposing table are opposed to each other. A lamp 108 serving as a light source for the exposure is disposed under the exposing table 102, and three filters 110R, 110G and 110B for red, green and blue, respectively, are disposed above the lamp 108 such that they may selectively cover the lamp 108. Thus, the lamp 108 is first covered by the filter 110R for red, and the lamp 108 and the filter 110R are integrally moved from a left end to a right end of the exposing table 102. Only light of a red component is selected from light of the lamp 108 by the filter 110R. The red component light passes through a portion of the mask member 115 at which no image is formed, that is, through a sheet portion of the base material, but is intercepted by a black image portion of the mask member 115. Consequently, an exposed face of the photosensitive recording medium 106 is sensitized by red light in accordance with an image of the mask member 115 so that a latent image for red is formed thereon. After completion of such processing, the mask member 115 for red is fed from the exposing table 102 in a rightward direction.
Simultaneously with such storage of the mask member 115 for red, the mask member 115 for green is inserted into the image forming apparatus. After the mask member 115 for green comes to the position at which it is opposed to the photosensitive recording medium 106, the lamp 108 is moved from the left end to the right end of the exposing table 102 while it is kept covered by the filter 110G for green, thereby performing an exposure operation with green light. The mask member 115 for green is then fed from the exposing table in a rightward direction.
Simultaneously, the mask member 115 for blue is inserted and transported to the exposing table 102, and an exposing operation of the photosensitive recording medium 106 with blue light is performed in a similar manner.
Latent images for red, green and blue are thus formed in an overlapping relationship on the same surface of the photosensitive recording medium 106 by multiple exposures during the three exposing operations.
A sensor 141 of the positioning unit 131 is aligned with the mask member 115 at a predetermined position of the exposing table 102. The sensor 141 includes a reflection-type light sensor which reads a positioning mark printed on the mask member 115 adjacent to the pair of rollers 132. The rollers 132 are rotated for feeding the mask member 115 in a feeding direction. The sensor 141 is installed on an installation board 142.
In this case, the pair of rollers 132 are rotated to feed the mask member 115, and rotation of the rollers 132 is controlled on the basis of sensing signals output from the sensor 141 while the sensor 141 is reading the positioning mark (not shown) of the mask member 115. Accordingly, the mask member 115 is fed to a predetermined position of the exposing table 102 by the rollers 132.
Use of a device of this type, however, deteriorates the accuracy of the positioning of the mask member 115 with respect to the sensor 141, when the smoothness of the mask member 115 is spoiled by a curl or a distortion appearing on the mask member 115. The cause of this problem is that the sensor 141 does not correctly output the output signal when either the distance between the sensor 141 and the positioning mark on the mask member 115 is changed or when the signal output from the sensor 141 for sensing the positioning mark is changed.